Crystal laminate structure

ABSTRACT

[Problem] To provide a crystal laminate structure having a β-Ga2O3 based single crystal film in which a dopant is included throughout the crystal and the concentration of the dopant can be set across a broad range. [Solution] In one embodiment of the present invention, provided is a crystal laminate structure 1 which includes: a Ga2O3 based substrate 10; and a β-Ga2O3 based single crystal film 12 formed by epitaxial crystal growth on a primary face 11 of the Ga2O3 based substrate 10 and including Cl and a dopant doped in parallel with the crystal growth at a concentration of 1×1013 to 5.0×1020 atoms/cm3.

The present application is a continuation application filed under 35 USC§ 120 claiming priority to co-pending application U.S. Ser. No.15/559,167, filed Sep. 18, 2017, which claims priority as a nationalstage filing under 35 USC § 371 of PCT/JP2016/054620 filed Feb. 17,2016, the entire contents of each of which are incorporated herein byreference.

TECHNICAL FIELD

The invention relates to a crystal laminate structure.

BACKGROUND ART

Conventionally, a method in which a dopant is added during crystalgrowth by the MBE (Molecular Beam Epitaxy) method or the EFG(Edge-defined Film-fed Growth) method (see, e.g., PTLs 1 and 2) and amethod in which a dopant is added to a grown β-Ga₂O₃-based singlecrystal by ion implantation (see, e.g., PTL 3) are known to dope aβ-Ga₂O₃-based single crystal.

CITATION LIST Patent Literature

-   [PTL 1]-   JP-A-2013-56803-   [PTL 2]-   JP-A-2013-82587-   [PTL 3]-   JP-A-2013-58637

SUMMARY OF INVENTION Technical Problem

When using the MBE method, however, doping with a concentration of notless than 1×10¹⁸ atoms/cm³ is difficult since impurities segregateduring epitaxial crystal growth. In addition, a doping profile in adepth direction and distribution uniformity on the wafer surface are notgood due to the impurity segregation and repeatability is poor.

Meanwhile, when using the EFG method, since a raw material contains animpurity at a concentration of about 1×10¹⁷ atoms/cm³, doping with alower concentration is not possible.

In case of ion implantation, implantation depth of the impurity ion islimited to about 1 μm. In addition, since the ion beam damages thecrystal, vacancy/void-type defects are introduced, causing deteriorationof crystallinity.

Therefore, it is one of objects of the invention to provide a crystallaminate structure having a β-Ga₂O₃-based single crystal film in which adopant is included throughout the crystal and the concentration of thedopant can be set across a broad range.

Solution to Problem

To achieve the above-mentioned object, an aspect of the inventionprovides a crystal laminate structure defined by [1] to [10] below.

[1] A crystal laminate structure, comprising: a Ga₂O₃-based substrate;and a β-Ga₂O₃-based single crystal film formed by epitaxial crystalgrowth on a principal surface of the Ga₂O₃-based substrate, wherein theβ-Ga₂O₃-based single crystal film comprises Cl and a dopant doped inparallel with the crystal growth at a concentration of not less than1×10¹³ atoms/cm³ and not more than 5.0×10²⁰ atoms/cm³.

[2] The crystal laminate structure defined by [1], wherein aconcentration of Cl of the β-Ga₂O₃-based single crystal film is not morethan 5×10¹⁶ atoms/cm³.

[3] The crystal laminate structure defined by [1] or [2], wherein thedopant comprises Si.

[4] The crystal laminate structure defined by [1] or [2], wherein thedopant concentration is not less than 6.5×10¹⁵ atoms/cm³ and not morethan 2.1×10²⁰ atoms/cm³.

[5] The crystal laminate structure defined by [1] or [2], wherein adensity of a carrier generated by the doping of the dopant is not lessthan 1×10¹³ cm⁻³ and not more than 5.0×10²⁰ cm⁻³.

[6] The crystal laminate structure defined by [5], wherein a density ofa carrier generated by the doping of the dopant is not less than3.2×10¹⁵ cm ⁻³ and not more than 1.8×10¹⁸ cm ⁻³.

[7] The crystal laminate structure defined by [1] or [2], wherein theβ-Ga₂O₃-based single crystal film comprises a β-Ga₂O₃ crystal film.

[8] The crystal laminate structure defined by [1] or [2], wherein theprincipal surface of the Ga₂O₃-based substrate has a plane orientationof (001), (010), (−201) or (101).

[9] The crystal laminate structure defined by [3], wherein a source gasof the dopant comprises a SiCl₄ gas.

[10] The crystal laminate structure defined by [1] or [2], wherein athickness of the β-Ga₂O₃-based single crystal film is not less than 1000nm.

Advantageous Effects of Invention

According to the invention, it is possible to provide a crystal laminatestructure having a β-Ga₂O₃-based single crystal film in which a dopantis included throughout the crystal and the concentration of the dopantcan be set across a broad range.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical cross-sectional view showing a crystal laminatestructure in an embodiment.

FIG. 2 is a vertical cross-sectional view showing a vapor phasedeposition system in the embodiment.

FIG. 3 is a graph which is obtained by thermal equilibrium calculationand shows a relation between equilibrium partial pressure of GaCl and anO₂/GaCl supplied partial pressure ratio when the atmosphere temperatureduring Ga₂O₃ crystal growth is 1000° C.

FIG. 4 is a graph showing a relation between R_(Si) and Si concentrationof a grown β-Ga₂O₃-based single crystal film.

FIG. 5 is a graph showing a relation between R_(Si), and carrier densityof the grown β-Ga₂O₃-based single crystal film.

FIG. 6 is a SIMS profile of Sn in a sample c4 shown in Table 3.

FIG. 7A is a SIMS profile of Si in a sample c3 shown in Table 3.

FIG. 7B is a SIMS profile of Si in the sample c4 shown in Table 3.

FIG. 7C is a SIMS profile of Si in a sample c6 shown in Table 3.

FIG. 7D is a SIMS profile of Si in a sample c7 shown in Table 3.

FIG. 8A is a SIMS profile of Cl in the sample c3 shown in Table 3.

FIG. 8B is a SIMS profile of Cl in the sample c4 shown in Table 3.

FIG. 8C is a SIMS profile of Cl in the sample c6 shown in Table 3.

FIG. 8D is a SIMS profile of Cl in the sample c7 shown in Table 3.

FIG. 9 is a graph showing a relation between carrier density andelectron mobility in the β-Ga₂O₃-based single crystal film.

DESCRIPTION OF EMBODIMENT Embodiment

(Configuration of Crystal Laminate Structure)

FIG. 1 is a vertical cross-sectional view showing a crystal laminatestructure 1 in an embodiment. The crystal laminate structure 1 has aGa₂O₃-based substrate 10 and a β-Ga₂O₃-based single crystal film 12formed on a principal surface 11 of the Ga₂O₃-based substrate 10 byepitaxial crystal growth.

The Ga₂O₃-based substrate 10 is a substrate formed of a Ga₂O₃-basedsingle crystal with a β-crystal structure. The Ga₂O₃-based singlecrystal here means a Ga₂O₃ single crystal or is a Ga₂O₃ single crystaldoped with an element such as Al or In, and may be, e.g., a(Ga_(x)Al_(y)In_((1-x-y)))₂O₃ (0<x≤1, 0≤y≤1, 0<x+y≤1) single crystalwhich is a Ga₂O₃ single crystal doped with Al and In. The band gap iswidened by adding Al and is narrowed by adding In. The Ga₂O₃-basedsubstrate 10 may also contain a conductive impurity such as Si.

The plane orientation of the principal surface 11 of the Ga₂O₃-basedsubstrate 10 is, e.g., (001), (010), (−201) or (101).

To form the Ga₂O₃-based substrate 10, for example, a bulk crystal of aGa₂O₃-based single crystal grown by, e.g., a melt-growth technique suchas the FZ (Floating Zone) method or the EFG (Edge Defined Film FedGrowth) method is sliced and the surface thereof is then polished.

The β-Ga₂O₃-based single crystal film 12 is formed of a Ga₂O₃-basedsingle crystal with a β-crystal structure in the same manner as theGa₂O₃-based substrate 10. The β-Ga₂O₃-based single crystal film 12 alsocontains a dopant such as Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Sc, Y,Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni,Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, B, Al, In, Tl, C, Si, Ge, Sn, Pb, N, P,As, Sb, Bi, S, Se or Te, etc., which is doped during crystal growth.

A concentration of the dopant contained in the β-Ga₂O₃-based singlecrystal film 12 is not less than 1×10¹³ atoms/cm³ and not more than5×10²⁰ atoms/cm³, preferably, not less than 6.5×10¹⁵ atoms/cm³ and notmore than 2.1×10²⁰ atoms/cm³.

A density of carrier generated by the doping of the dopant is, e.g., notless than 3.2×10¹⁵ cm⁻³ and not more than 1.8×10¹⁸ cm⁻³.

The β-Ga₂O₃-based single crystal film 12 also contains Cl at aconcentration of not more than 5×10¹⁶ atoms/cm³. This results from thatthe β-Ga₂O₃-based single crystal film 12 is formed by the HVPE methodusing Cl-containing gas. Generally, Cl-containing gas is not used toform aβ-Ga₂O₃-based single crystal film when using a method other thanthe HVPE method, and the β-Ga₂O₃-based single crystal film does notcontain Cl, or at least does not contain 1×10¹⁶ cm ⁻³ or more of Cl.

Meanwhile, the β-Ga₂O₃-based single crystal film 12 is formed by theHYPE (Halide Vapor Phase Epitaxy) method with a high crystal growthrate, and thus can be formed thick, e.g., can have a thickness of notless than 1000 nm. In general, a growth rate of the β-Ga₂O₃-based singlecrystal film by industrial HVPE is 200 μm/h, and in this case, a film ofup to 1000 μm in thickness can be grown in a realistic production time.In other words, it is possible to form the β-Ga₂O₃-based single crystalfilm 12 having a thickness of not less than 1000 nm and not more than1000 μm. In this regard, use of the MBE method is not realistic inactual production since a crystal growth rate of the β-Ga₂O₃-basedsingle crystal film is about 120 nm/h and it requires not less than 8hours to form a film of not less than 1000 nm in thickness.

Additionally, a buffer layer may be provided between the Ga₂O₃-basedsubstrate 10 and the β-Ga₂O₃-based single crystal film 12 to preventcarrier compensation in the β-Ga₂O₃-based single crystal film 12 due toimpurity diffusion from the Ga₂O₃-based substrate 10.

(Structure of Vapor Phase Deposition System)

A structure of a vapor phase deposition system used for growing theβ-Ga₂O₃-based single crystal film 12 in the present embodiment will bedescribed below as an example.

FIG. 2 is a vertical cross-sectional view showing a vapor phasedeposition system 2 in the embodiment. The vapor phase deposition system2 is a vapor phase deposition system using the HVPE method, and has areaction chamber 20 having a first gas introducing port 21, a second gasintroducing port 22, a third gas introducing port 23, a fourth gasintroducing port 24 and an exhaust port 25, and a first heating means 27and a second heating means 28 which are placed to surround the reactionchamber 20 to heat predetermined regions in the reaction chamber 20.

The growth rate when using the HVPE method is higher than that in theMBE method, etc. In addition, in-plane distribution of film thickness ishighly uniform and it is possible to grow a large-diameter film.Therefore, it is suitable for mass production of crystal.

The reaction chamber 20 has a source reaction region R1 in which areaction container 26 containing a Ga source is placed and a galliumsource gas is produced, and a crystal growth region R2 in which theGa₂O₃-based substrate 10 is placed and the β-Ga₂O₃-based single crystalfilm 12 is grown thereon. The reaction chamber 20 is formed of, e.g.,quartz glass.

Here, the reaction container 26 is formed of, e.g., quartz glass and theGa source contained in the reaction container 26 is metal gallium.

The first heating means 27 and the second heating means 28 are capableof respectively heating the source reaction region R1 and the crystalgrowth region R2 of the reaction chamber 20. The first heating means 27and the second heating means 28 are, e.g., resistive heaters orradiation heaters.

The first gas introducing port 21 is a port for introducing aCl-containing gas (Cl₂ gas or HCl gas) into the source reaction regionR1 of the reaction chamber 20 using an inert carrier gas (N₂ gas, Ar gasor He gas). The second gas introducing port 22 is a port for introducingan oxygen-containing gas (O₂ gas or H₂O gas, etc.) as an oxygen sourcegas into the crystal growth region R2 of the reaction chamber 20 usingan inert carrier gas (N₂ gas, Ar gas or He gas). The third gasintroducing port 23 is a port for introducing an inert carrier gas (N₂gas, Ar gas or He gas) into the crystal growth region R2 of the reactionchamber 20. The fourth gas introducing port 24 is a port for introducinga source gas of dopant to be added to the β-Ga₂O₃-based single crystalfilm 12, such as Si (e.g., silicon tetrachloride, etc.), into thecrystal growth region R2 of the reaction chamber 20 using an inertcarrier gas (N₂ gas, Ar gas or He gas).

(Growth of β-Ga₂O₃-Based Single Crystal Film)

To grow the β-Ga₂O₃-based single crystal film 12, it is possible to usea technique of growing β-Ga₂O₃-based single crystal film disclosed inJapanese Patent Application No. 2014-088589. A process of growing theβ-Ga₂O₃-based single crystal film 12 in the present embodiment will bedescribed below as an example.

Firstly, the source reaction region R1 of the reaction chamber 20 isheated by the first heating means 27 and an atmosphere temperature inthe source reaction region R1 is then maintained at a predeterminedtemperature.

Next, in the source reaction region R1, a Cl-containing gas introducedthrough the first gas introducing port 21 using a carrier gas is reactedwith the metal gallium in the reaction container 26 at theabove-mentioned atmosphere temperature, thereby producing a galliumchloride gas.

The atmosphere temperature in the source reaction region R1 here ispreferably a temperature at which GaCl gas has the highest partialpressure among component gases of the gallium chloride gas produced bythe reaction of the metal gallium in the reaction container 26 with theCl-containing gas. The gallium chloride gas here contains GaCl gas,GaCl₂ gas, GaCl₃ gas and (GaCl₃)₂ gas, etc.

The temperature at which a driving force for growth of Ga₂O₃ crystal ismaintained is the highest with the GaCl gas among the gases contained inthe gallium chloride gas. Growth at a high temperature is effective toobtain a high-quality Ga₂O₃ crystal with high purity. Therefore, forgrowing the β-Ga₂O₃-based single crystal film 12, it is preferable toproduce a gallium chloride gas in which a partial pressure of GaCl gashaving a high driving force for growth at a high temperature is high.

It is possible to increase a partial pressure ratio of the GaCl gas inthe gallium chloride gas by reacting the metal gallium with theCl-containing gas at an atmosphere temperature of about not less than300° C. Therefore, it is preferable that the metal gallium in thereaction container 26 be reacted with the Cl-containing gas in a statethat the atmosphere temperature in the source reaction region R1 ismaintained at not less than 300° C. by using the first heating means 27.

At the atmosphere temperature of, e.g., 850° C., the partial pressureratio of the GaCl gas is predominantly high (the equilibrium partialpressure of the GaCl gas is four orders of magnitude greater than theGaCl₂ gas and is eight orders of magnitude greater than the GaCl₃ gas)and the gases other than GaCl gas hardly contribute to the growth ofGa₂O₃ crystal.

Meanwhile, considering the lifetime of the first heating means 27 andheat resistance of the reaction chamber 20 formed of quartz glass, etc.,it is preferable that the metal gallium in the reaction container 26 bereacted with the Cl-containing gas in a state that the atmospheretemperature in the source reaction region R1 is maintained at not morethan 1000° C.

In addition, if hydrogen is contained in an atmosphere for growing theβ-Ga₂O₃-based single crystal film 12, surface flatness and a drivingforce for growth of the β-Ga₂O₃-based single crystal film 12 decrease.Therefore, it is preferable that a Cl₂ gas not containing hydrogen beused as the Cl-containing gas.

Next, in the crystal growth region R2, the gallium chloride gas producedin the source reaction region R1 is mixed with the oxygen-containing gasintroduced through the second gas introducing port 22 and the dopantsource gas such as Si introduced through the fourth gas introducing port24, the Ga₂O₃-based substrate 10 is then exposed to the mixed gas, andthe β-Ga₂O₃-based single crystal film 12 containing the dopant isthereby epitaxially grown on the Ga₂O₃-based substrate 10. At this time,in a furnace housing the reaction chamber 20, pressure in the crystalgrowth region R2 is maintained at, e.g., 1 atm.

The dopant source gas used here is preferably a chloride-based gas toprevent other unintentional impurities from being mixed. For example,when Si, Ge, Sn or Pb (Group 14 element) is selected from the dopantslisted above and is used as a dopant, a chloride-based gas such asSiCl₄, GeCl₄, SnCl₄ or PbCl₂, respectively, is used. Here, thechloride-based gas is not limited to a compound of an element with onlychlorine, and may be, e.g., a silane-based gas such as SiHCl₃.

The dopant such as Si is doped during growth of a β-Ga₂O₃-based singlecrystal.

If hydrogen is contained in an atmosphere for growing the β-Ga₂O₃-basedsingle crystal film 12, surface flatness and a driving force for growthof the β-Ga₂O₃-based single crystal film 12 decrease. Therefore, it ispreferable that an O₂ gas not containing hydrogen be used as theoxygen-containing gas.

Meanwhile, the smaller the equilibrium partial pressure of the GaCl gas,the more the GaCl gas is consumed for growth of Ga₂O₃ crystal and theGa₂O₃ crystal grows efficiently. For example, the equilibrium partialpressure of the GaCl gas sharply falls when a ratio of the suppliedpartial pressure of the O₂ gas to the supplied partial pressure of theGaCl gas (the O₂/GaCl supplied partial pressure ratio) is not less than0.5. Therefore, to efficiently grow the β-Ga₂O₃-based single crystalfilm 12, the β-Ga₂O₃-based single crystal film 12 is preferably grown ina state that the O₂/GaCl supplied partial pressure ratio in the crystalgrowth region R2 is not less than 0.5.

FIG. 3 is a graph which is obtained by thermal equilibrium calculationand shows a relation between an equilibrium partial pressure of GaCl andan O₂/GaCl supplied partial pressure ratio when the atmospheretemperature during Ga₂O₃ crystal growth is 1000° C. It is calculatedusing the supplied partial pressure value of the GaCl gas fixed to1×10⁻³ atm, a furnace pressure of 1 atom adjusted by using, e.g., aninert gas such as N₂ as a carrier gas, and various values of the O₂ gassupplied partial pressure.

In FIG. 3, the horizontal axis indicates the O₂/GaCl supplied partialpressure ratio and the vertical axis indicates an equilibrium partialpressure (atm) of GaCl gas. It is shown that the smaller the equilibriumpartial pressure of the GaCl gas, the more the GaCl gas is consumed forgrowth of Ga₂O₃ crystal, i.e., the Ga₂O₃ crystal grows efficiently.

FIG. 3 shows that the equilibrium partial pressure of the GaCl gassharply falls at the O₂/GaCl supplied partial pressure ratio of not lessthan 0.5.

Meanwhile, a growth temperature of not less than 900° C. is required togrow the β-Ga₂O₃-based single crystal film 12. A single crystal may notbe obtained at less than 900° C.

(Evaluation of β-Ga₂O₃-Based Single Crystal Film)

Plural β-Ga₂O₃-based single crystal films 12 were formed under differentgrowth conditions, and Si concentration and carrier density thereof weremeasured. The results are shown in Tables 1 to 3 below.

Samples a1 and a2 shown in Table 1 are the crystal laminate structures 1each having the β-Ga₂O₃-based single crystal film 12 formed under thecondition that a partial pressure of the GaCl gas supplied as a Gasource gas, P⁰ _(GaCl), was fixed to 2×10⁻⁴ atm. Meanwhile, samples b1to b12 shown in Table 2 are the crystal laminate structures 1 eachhaving the β-Ga₂O₃-based single crystal film 12 formed under thecondition that P⁰ _(GaCl) was fixed to 5×10⁻⁴ atm, and samples c1 to c9shown in Table 3 are the crystal laminate structures 1 each having theβ-Ga₂O₃-based single crystal film 12 formed under the condition that P⁰_(GaCl) was fixed to 1×10⁻³ atm.

In the samples a1, a2, b1 to b12 and c1 to c9, a 7 μm-thickSi-containing β-Ga₂O₃ single crystal film was formed as theβ-Ga₂O₃-based single crystal film 12 on a β-Ga₂O₃ substrate having a(001)-oriented principal surface used as the Ga₂O₃-based substrate 10.

“R_(Si)” in Tables 1 to 3 is a physical quantity expressed by P⁰_(SiCl4)/(P⁰ _(GaCl)+P⁰ _(SiCl4)). P⁰ _(SiCl4) here is a partialpressure of the SiCl₄ gas supplied as a Si source gas during growth ofthe β-Ga₂O₃-based single crystal film 12. The value of P⁰ _(GaCl) forforming the β-Ga₂O₃-based single crystal film 12 is not specificallylimited as long as pressure in the crystal growth region R2 of the vaporphase deposition system 2 (e.g., 1 atm) is maintained.

“Si concentration” in Tables 1 to 3 is a concentration of Si as a dopantin the β-Ga₂O₃-based single crystal film 12 (in a region at a depth of 2to 6 μm from the surface) obtained by SIMS (Secondary Ion MassSpectrometry). The background level of the Si concentration measured bySIMS differs depending on the measurement conditions. The Siconcentration of the samples b2 and b3 was measured under the conditionthat the background level was about 3.0×10¹⁵ atoms/cm³, the Siconcentration of the other samples was measured under the condition thatthe background level was about 3.0×10¹⁶ atoms/cm³, and Si at aconcentration of not more than the background level is not detectable ineach sample.

“N_(d)-N_(a)” in Tables 1 to 3 is a difference between a donorconcentration N_(d) and an acceptor concentration N_(a), i.e., a carrierconcentration, obtained by ECV (Electrochemical Capacitance-Voltage)measurement.

TABLE 1 N_(d) − N_(a) Sample number R_(Si) Si concentration [atoms/cm³][cm⁻³] a1 6.3 × 10⁻⁶ 7.1 × 10¹⁷ — a2 3.1 × 10⁻⁵ 2.1 × 10¹⁸ —

TABLE 2 Sample number R_(Si) Si concentration [atoms/cm³] N_(d) − N_(a)[cm⁻³] b1 2.5 × 10⁻⁸ — 7.4 × 10¹⁵ b2 2.5 × 10⁻⁸ 6.5 × 10¹⁵ — b3 5.0 ×10⁻⁸ 1.0 × 10¹⁶ — b4 2.5 × 10⁻⁷ — 3.6 × 10¹⁶ b5 2.5 × 10⁻⁶ 1.7 × 10¹⁷ —b6 2.5 × 10⁻⁶ 2.2 × 10¹⁷ — b7 5.0 × 10⁻⁶ 3.5 × 10¹⁷ 2.8 × 10¹⁷ b8 1.0 ×10⁻⁵ 5.7 × 10¹⁷ 5.7 × 10¹⁷ b9 1.3 × 10⁻⁵ 5.3 × 10¹⁷ 5.2 × 10¹⁷ b10 1.3 ×10⁻⁵ 5.7 × 10¹⁷ — b11 6.3 × 10⁻⁵ 2.7 × 10¹⁸ — b12 7.5 × 10⁻⁴ 1.4 × 10²⁰—

TABLE 3 Sample number R_(Si) Si concentration [atoms/cm³] N_(d) − N_(a)[cm⁻³] c1 1.3 × 10⁻⁸ — 3.4 × 10¹⁵ c2 1.3 × 10⁻⁷ — 2.0 × 10¹⁶ c3 1.3 ×10⁻⁶ 8.8 × 10¹⁶ — c4 2.5 × 10⁻⁶ 1.5 × 10¹⁷ — c5 3.1 × 10⁻⁶ 1.2 × 10¹⁷1.9 × 10¹⁷ c6 6.3 × 10⁻⁶ 2.2 × 10¹⁷ — c7 1.3 × 10⁻⁵ 4.0 × 10¹⁷ — c8 3.1× 10⁻⁵ 1.2 × 10¹⁸ 1.8 × 10¹⁸ c9 1.9 × 10⁻³ 2.1 × 10²⁰ —

From Tables 2 and 3, it is understood that the carrier density obtainedby ECV measurement substantially coincides with the Si concentrationobtained by SIMS measurement. From Table 4 which is described later, itis understood that the carrier density obtained by Hall measurement alsosubstantially coincides with the Si concentration obtained by SIMSmeasurement.

FIG. 4 is a graph which is obtained from the data of Tables 1 to 3 andTable 4 (described later) and shows a relation between R_(Si) and Siconcentration of the grown β-Ga₂O₃-based single crystal film 12.

In FIG. 4, plot symbols “⋄” indicate the measured values for the samplesa1 and a2 shown in Table 1, “◯” indicate the measured values for thesamples b5 to b12 shown in Table 2, and “□” indicate the measured valuesfor the samples c3 to c9 shown in Table 3. Plot symbols “●” in FIG. 4indicate the measured values for the samples d3 and d4 shown in Table 4.

FIG. 4 shows that the relation between R_(Si) and the Si concentrationof the β-Ga₂O₃-based single crystal film 12 is a substantially linearrelation and the Si concentration of the β-Ga₂O₃-based single crystalfilm 12 can be controlled by adjusting the SiCl₄ gas supply amountrelative to the GaCl gas supply amount.

FIG. 5 is a graph showing a relation between R_(Si) and carrier densityof the grown β-Ga₂O₃-based single crystal film 12.

In FIG. 5, plot symbols “◯” indicate the measured values for the samplesb1, b4 and b7 to b9 shown in Table 2, and “□” indicate the measuredvalues for the samples c1, c2, c5 and c8 shown in Table 3. Plot symbols“●” in FIG. 5 indicate the measured values for the samples d1 to d4shown in Table 4 (described later).

FIG.5 shows that the relation between R_(Si) and the carrier density ofthe β-Ga₂O₃-based single crystal film 12 is a substantially linearrelation.

From the actual measurement data shown in FIG. 4, it is understood thatit is possible to control the Si concentration at least in the range ofnot less than 6.5×10¹⁵ atoms/cm³ and not more than 2.1×10²⁰ atoms/cm³.Furthermore, based on the linearity of the actual measurement data shownin FIG. 4, it is considered that the Si concentration can be controlledin a wider range.

However, when trying to dope Si at a concentration of less than about1×10¹³ atoms/cm³, it is difficult to accurately control the partialpressure of the doping gas and it is thus difficult to control the Siconcentration. Meanwhile, the upper limit of the Si concentration atwhich crystal quality of the β-Ga₂O₃-based single crystal film 12 can bemaintained by solid solubility of Si in β-Ga₂O₃-based single crystal isabout 5.0×10²⁰ atoms/cm³. Thus, it is preferable to control the Siconcentration in the range of not less than 1×10¹³ atoms/cm³ and notmore than 5.0×10²⁰ atoms/cm³.

Meanwhile, from the actual measurement data shown in FIG. 5, it isunderstood that it is possible to control the carrier density at leastin the range of not less than 3.2×10¹⁵ cm⁻³ and not more than 1.8×10¹⁸cm⁻³ by adding the dopant. Furthermore, based on the linearity of theactual measurement data shown in FIG. 5, it is considered that thecarrier density can be controlled in a wider range, in the same manneras the Si concentration.

In an undoped β-Ga₂O₃-based single crystal film grown by the HVPEmethod, carriers with a density of not more than 1×10¹³ cm ⁻³ remains asdisclosed in Japanese Patent Application No. 2014-088589, and it isdifficult to control the carrier concentration to be lower than 1×10¹³cm⁻³. In addition, the upper limit of the Si concentration at whichcrystal quality of the β-Ga₂O₃-based single crystal film 12 can bemaintained is about 5.0×10²⁰ atoms/cm³ as described above. Therefore,for the carrier concentration which substantially coincides with the Siconcentration, the upper limit at which crystal quality of theβ-Ga₂O₃-based single crystal film 12 can be maintained is considered tobe about 5.0×10²⁰ cm⁻³. Thus, it is preferable to control the carrierdensity of the Ga₂O₃-based single crystal film in the range of not lessthan 1×10¹³ cm⁻³ and not more than 5.0×10²⁰ cm ⁻³.

FIG. 6 is a SIMS profile of Sn in the sample c4 shown in Table 3. InFIG. 6, the horizontal axis indicates a depth from the surface of thecrystal laminate structure 1 (from the surface of the β-Ga₂O₃-basedsingle crystal film 12), and the vertical axis indicates Snconcentration. In addition, a dotted line in FIG. 6 indicates thebackground level (2.0×10¹⁷ atoms/cm³) of the Sn concentration.

Sn is a dopant contained in the Ga₂O₃-based substrate 10 of the samplec4. From the SIMS profile of Sn in FIG. 6, it is understood that aninterface between the Ga₂O₃-based substrate 10 and the β-Ga₂O₃-basedsingle crystal film 12 is located at a depth about of 7 μm from thesurface of the crystal laminate structure 1. Since the β-Ga₂O₃-basedsingle crystal films 12 of all samples shown in Table 1 to 3 were formedunder the same growth conditions, an interface between the Ga₂O₃-basedsubstrate 10 and the β-Ga₂O₃-based single crystal film 12 in each sampleis located at a depth of about 7 μm from the surface of the crystallaminate structure 1.

FIGS. 7A to 7D are SIMS profiles of Si in the samples c3, c4, c6 and c7shown in Table 3. In FIGS. 7A to 7D, the horizontal axis indicates adepth from the surface of the crystal laminate structure 1, and thevertical axis indicates Si concentration. In addition, in FIGS. 7A to7D, a horizontal dotted line indicates the background level of the Siconcentration (3.0×10¹⁶ atoms/cm³) and a vertical dot-and-dash lineindicates the position of the interface between the Ga₂O₃-basedsubstrate 10 and the β-Ga₂O₃-based single crystal film 12 (depth of 7μm).

FIGS. 8A to 8D are SIMS profiles of Cl in the samples c3, c4, c6 and c7shown in Table 3. In FIGS. 8A to 8D, the horizontal axis indicates adepth from the surface of the crystal laminate structure 1, and thevertical axis indicates Cl concentration. In addition, in FIGS. 8A to8D, a horizontal dotted line indicates the background level of the Clconcentration (2.0×10¹⁵ atoms/cm³) and a vertical dot-and-dash lineindicates the position of the interface between the Ga₂O₃-basedsubstrate 10 and the β-Ga₂O₃-based single crystal film 12 (depth of 7μm).

According to FIGS. 8A to 8D, the β-Ga₂O₃ single crystal film contains Clat a concentration of not more than about 5×10¹⁶ atoms/cm³. This resultsfrom that the Ga₂O₃ single crystal film is formed by the HVPE methodusing Cl-containing gas. Generally, Cl-containing gas is not used toform a Ga₂O₃ single crystal film when using a method other than the HVPEmethod, and the Ga₂O₃ single crystal film does not contain Cl, or atleast does not contain 1×10¹⁶ atoms/cm³ or more of Cl.

Plural β-Ga₂O₃ single crystal films were formed under different growthconditions, and carrier density, resistivity and electron mobilitythereof were measured. The results are shown in Table 4 below. Thesevalues were obtained by Hall measurement using the Van del Pauw method.In the Hall measurement, a DC magnetic field of 0.57 T and an electriccurrent of 0.1 mA were applied at room temperature (25° C.). Table 4also shows the Si concentration in the β-Ga₂O₃-based single crystal film(in a region at a depth of 2 to 6 μm from the surface) measured by SIMS.

The samples d1 to d4 to be subjected to Hall measurement were made asfollows. Firstly, a β-Ga₂O₃ single crystal film was epitaxially grown ona highly Fe-doped β-Ga₂O₃ substrate having a (001)-oriented principalsurface. At this time, SiCl₄ gas supply was started in the middle of thegrowth of the β-Ga₂O₃ single crystal film so that a 2 μm-thick undopedβ-Ga₂O₃ single crystal film and an 8 μm-thick Si-doped β-Ga₂O₃ singlecrystal film were formed. The undoped β-Ga₂O₃ single crystal film has afunction of preventing diffusion of Fe from the β-Ga₂O₃ substrate andthus preventing carrier compensation in the Si-doped β-Ga₂O₃ singlecrystal film.

The β-Ga₂O₃ single crystal film here was formed under the condition thatP⁰ _(GaCl) was fixed to 1×10⁻³ atm.

Next, for surface flattening, the surface of the Si-doped β-Ga₂O₃ singlecrystal film was polished 2 μm by CMP (Chemical Mechanical Polishing).Next, circular In electrodes having a diameter of 1 mm were formed atfour corners on the surface of the Si-doped β-Ga₂O₃ single crystal filmand were then annealed in a N₂ atmosphere at 900° C. for 90 seconds,thereby forming ohmic electrodes.

In the samples d1 to d4 to be subjected to Hall measurement, the β-Ga₂O₃substrate corresponds to the Ga₂O₃-based substrate 10 and the Si-dopedβ-Ga₂O₃ single crystal film corresponds to the β-Ga₂O₃-based singlecrystal film 12.

TABLE 4 Mobility Si con- Sample N_(d) − N_(a) Resistivity [cm²/centration number R_(Si) [cm⁻³] [Ω · cm] V · s] [atoms/cm³] d1 1.3 ×10⁻⁸ 3.2 × 10¹⁵ 12.5 1.60 × 10² — d2 1.3 × 10⁻⁷ 1.2 × 10¹⁶ 3.44 1.55 ×10² — d3 3.1 × 10⁻⁶ 2.2 × 10¹⁷ 2.47 × 10⁻¹ 1.14 × 10² 2.0 × 10¹⁷ d4 3.1× 10⁻⁵ 1.2 × 10¹⁸ 5.96 × 10⁻² 8.98 × 10 1.1 × 10¹⁸

FIG. 9 is a graph which is obtained from the data of Table 4 and shows arelation between carrier density (N_(d)-N_(a)) and electron mobility inthe β-Ga₂O₃-based single crystal film 12.

FIG. 9 shows that electron mobility increases with a decrease in thecarrier density of the β-Ga₂O₃-based single crystal film 12.

Although the β-Ga₂O₃ substrate having a (001)-oriented principal surfacewas used as the Ga₂O₃-based substrate 10 in the samples a1, a2, b1 tob12, c1 to c9 and d1 to d4 as described above, similar evaluationresults are obtained also when a β-Ga₂O₃ substrate having a (−201)-,(101)- or (010)-oriented, principal surface is used. In addition,similar evaluation results are obtained also when another β-Ga₂O₃-basedsubstrate is used instead of the β-Ga₂O₃ substrate or when anotherβ-Ga₂O₃-based single crystal film is formed instead of the β-Ga₂O₃single crystal film.

(Effects of the Embodiment)

In the embodiment, by adding a dopant while growing a β-Ga₂O₃-basedsingle crystal using the HVPE method, it is possible to control thedopant concentration of the β-Ga₂O₃-based single crystal in a widerrange than when using the MBE method or the EFG method. In addition,unlike when using ion implantation, problems such as limitation ofdopant implantation depth or deterioration of crystallinity due to ionbeam do not occur.

Although the embodiment of the invention has been described, theinvention is not intended to be limited to the embodiment, and thevarious kinds of modifications can be implemented without departing fromthe gist of the invention.

In addition, the invention according to claims is not to be limited tothe embodiment described above. Further, it should be noted that allcombinations of the features described in the embodiment are notnecessary to solve the problem of the invention.

INDUSTRIAL APPLICABILITY

Provided is a crystal laminate structure having a β-Ga₂O₃-based singlecrystal film in which a dopant is included throughout the crystal andthe concentration of the dopant can be set across a broad range.

REFERENCE SIGNS LIST

-   1 CRYSTAL LAMINATE STRUCTURE-   10 Ga₂O₃-BASED SUBSTRATE-   11 PRINCIPAL SURFACE-   12 β-Ga₂O₃-BASED SINGLE CRYSTAL FILM

The invention claimed is:
 1. A crystal laminate structure, comprising: aGa₂O₃-based substrate; a Si-doped β-Ga₂O₃-based single crystal epitaxialfilm grown on a principal surface of the Ga₂O₃-based substrate; whereinthe Si-doped β-Ga₂O₃-based single crystal epitaxial film contains Clresulting from HVPE method at a concentration of not more than 5×10¹⁶atoms/cm³, and wherein the Si-doped β-Ga₂O₃-based single crystalepitaxial film has an electron mobility from 8.98×10 to 1.60×10²cm²/V·sat 25° C.
 2. The crystal laminate structure, according to claim 1,wherein the Si-doped β-Ga₂O₃-based single crystal epitaxial film has acarrier density from 1.2×10¹⁸/cm³ to 3.2×10¹⁵/cm^(3.)
 3. The crystallaminate structure, according to claim 1, wherein the Si-dopedβ-Ga₂O₃-based single crystal epitaxial film has a resistivity from5.96×10⁻²Ω·cm to 12.5Ω·cm.